Laser processing

ABSTRACT

The invention provides a system and method for vaporizing a target structure on a substrate. According to the invention, a calculation is performed, as a function of wavelength, of an incident beam energy necessary to deposit unit energy in the target structure. Then, for the incident beam energy, the energy expected to be deposited in the substrate as a function of wavelength is calculated. A wavelength is identified that corresponds to a relatively low value of the energy expected to be deposited in the substrate, the low value being substantially less than a value of the energy expected to be deposited in the substrate at a higher wavelength. A laser system is provided configured to produce a laser output at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate. The laser output is directed at the target structure on the substrate at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate, in order to vaporize the target structure.

BACKGROUND OF THE INVENTION

[0001] This invention relates to laser processing systems and methods,including systems and methods for removing, with high yield,closely-spaced metal link structures or “fuses” on a silicon substrateof an integrated circuit or memory device.

[0002] Laser systems can be employed to remove fuse structures (“blowlinks”) in integrated circuits and memory devices such as ASICs, DRAMs,and SRAMs, for purposes such as removing defective elements andreplacing them with redundant elements provided for this purpose(“redundant memory repair”), or programming of logic devices. Linkprocessing laser systems include the M320 and M325 systems manufacturedby General Scanning, Inc, which produce laser outputs over a variety ofwavelengths, including 1.047 μm, 1.064 μm, and 1.32 μm.

[0003] Economic imperatives have led to the development of smaller, morecomplex, higher-density semiconductor structures. These smallerstructures can have the advantage of operation at relatively high speed.Also, because the semiconductor device part can be smaller, a greaternumber of parts can be included in a single wafer. Because the cost ofprocessing a single wafer in a semiconductor fabrication plant can bealmost independent of the number of parts on the wafer, the greaternumber of parts per wafer can translate into lower cost per part.

[0004] In the 1980s, semiconductor device parts often includedpolysilicon or silicide interconnects. Although poly-based interconnectsare relatively poor conductors, they were easily fabricated usingprocesses available at the time, and were well-suited to the wavelengthsgenerated by the Nd:YAG lasers commonly available at the time. Asgeometries shrank, however, the poor conductivity of polysiliconinterconnects and link structures became problematic, and somesemiconductor manufacturers switched to aluminum. It was found thatcertain conventional lasers did not cut the aluminum links as well asthey had cut polysilicon links, and in particular that damage to thesilicon substrate could occur. This situation could be explained by thefact that the reflection in aluminum is very high and the absorption islow. Therefore, increased energy must be used to overcome this lowabsorption. The higher energy can tend to damage the substrate when toomuch energy is used.

[0005] Sun et al., U.S. Pat. No. 5,265,114 advances an “absorptioncontrast” model for selecting an appropriate laser wavelength to cutaluminum and other metals such as nickel, tungsten, and platinum. Inparticular, this patent describes selecting a wavelength range in whichsilicon is almost transparent and in which the optical absorptionbehavior of the metal link material is sufficient for the link to beprocessed. The patent states that the 1.2 to 2.0 Am wavelength rangeprovides a high absorption contrast between a silicon substrate andhigh-conductivity link structures, as compared with laser wavelengths of1.064 μm and 0.532 μm.

SUMMARY OF THE INVENTION

[0006] The invention provides a system and method for vaporizing atarget structure on a substrate. According to the invention, acalculation is performed, as a function of wavelength, of an incidentbeam energy necessary to deposit unit energy in the target structure.Then, for the incident beam energy, the energy expected to be depositedin the substrate as a function of wavelength is calculated. A wavelengthis identified that corresponds to a relatively low value of the energyexpected to be deposited in the substrate, the low value beingsubstantially less than a value of the energy expected to be depositedin the substrate at a higher wavelength. A laser system is providedconfigured to produce a laser output at the wavelength corresponding tothe relatively low value of the energy expected to be deposited in thesubstrate. The laser output is directed at the target structure on thesubstrate at the wavelength corresponding to the relatively low value ofthe energy expected to be deposited in the substrate, in order tovaporize the target structure.

[0007] Certain applications of the invention involve selection of awavelength appropriate for cutting a metal link without producingunacceptable damage to a silicon substrate, where the wavelength is lessthan, rather than greater than, the conventional wavelengths of 1.047 μmand 1.064 μm. This method of wavelength selection is advantageousbecause the use of shorter wavelengths can result in smaller laserspots, other things being equal, and hence greater ease in hitting onlythe desired link with the laser spot. In particular, other things beingequal, laser spot size is directly proportional to wavelength accordingto the formula: spot size is proportional to λf, where λ is the laserwavelength and f is the f-number of the optical system.

[0008] Moreover, certain applications of the invention involve selectionof a wavelength at which a substrate has low absorption but aninterconnect material has higher absorption than at conventionalwavelengths of 1.047 μm and 1.064 μm or higher-than-conventionalwavelengths. Because of the reduced reflectivity of the interconnectmaterial, the incident laser energy can be reduced while theinterconnect material nevertheless absorbs sufficient energy for theinterconnect to be blown without multiple laser pulses (which can impactthroughput) or substantial collateral damage due to the laser beam.

[0009] The invention can effect high-quality laser link cuts onhigh-conductivity interconnect materials such as copper, gold, and thelike, arranged in closely-spaced patterns, with only a single laserpulse, and without damaging the substrate. The invention can furtherallow a smaller laser spot size than would be obtainable at wavelengthsof 1.047 μm, 1.064 μm, or higher, while still providing acceptable linkcuts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram of a laser system according to theinvention for removing a link of a semiconductor device, where the linkis manufactured of a material such as copper or gold.

[0011]FIG. 2 is a perspective diagrammatic view of a link on a substrateof a semiconductor device.

[0012]FIG. 3 is plot of absorption of copper, gold, aluminum, andsilicon as a function of wavelength.

[0013]FIG. 4 is a plot of an substrate absorption function according tothe invention, for copper, gold, and aluminum links on a siliconsubstrate, as a function of wavelength.

[0014]FIG. 5 is a plot of the function L−S for copper, gold, andaluminum links on a silicon substrate, where L is the absorption in thelink and S is the absorption in the substrate.

[0015]FIG. 6 is a plot of the function (L−S)/(L+S) for copper, gold, andaluminum links on a silicon substrate, where L is the absorption in thelink and S is the absorption in the substrate.

DETAILED DESCRIPTION

[0016] In the block diagram of FIG. 1, a system for removing a link of asemiconductor device is shown. Laser 10 is constructed to operate at aconventional wavelength such as 1.047 μm. It is aligned to a laseroutput system that includes a wavelength shifter 12, such as a frequencydoubler or an optical parametric oscillator (OPO), constructed to shiftto a wavelength less than 0.55 μm, in the “green” region of thewavelength spectrum. As is explained in more detail below, the beam isthen passed through the remainder of the laser output system, includinga controlled electro-acousto-optic attenuator 13, a telescope 14 thatexpands the beam, and, a scanning head 15, that scans the beam over afocusing lens 16 by means of two scanner galvanometers, 18 and 20. Thespot is focused onto wafer 22 for removing links 24, under control ofcomputer 33.

[0017] The laser 10 is mounted on a stable platform 11 relative to thegalvanometers and the work piece. It is controlled from outside of thelaser itself by computer 33 to transmit its beam to the scanner headcomprising the accurate X and Y galvanometers 18 and 20. It is veryimportant, in removing links that the beam be positioned with accuracyof less than {fraction (3/10)} of a micron. The timing of the laserpulse to correlate with the position of the continually movinggalvanometers is important. The system computer 33 asks for a laserpulse on demand.

[0018] A step and repeat table 34 moves the wafer into position to treateach semiconductor device.

[0019] In one embodiment, the laser 10 is a neodymium vanadate laser,with an overall length L of about 6 inches, and a short cavity length.

[0020] The shifter 12 of this preferred embodiment is external to thecavity, and is about another 4 inches long. In alternative embodiments,laser 10 can be configured to produce a laser output having anappropriate wavelength, so that no shifter would be required.

[0021] The laser is a Q-switched diode pumped laser, of sufficientlength and construction to enable external control of pulse rate withhigh accuracy by computer 33.

[0022] The cavity of the laser includes a partially transmissive mirror7, optimized at the wavelength at which the lasing rod 6 of neodymiumvanadate is pumped by the diode. The partially transmissive outputmirror 9 is also optimized at this wavelength.

[0023] The pumping diode 4 produces between about one and two wattsdepending on the design. It focuses onto the rear of the laser rod 6. Asmentioned, the laser rod is coated, on its pumped end, with a mirror 7appropriate for the standard laser wavelength of 1.064 μm or 1.047 μm.The other end of the rod is coated with a dichroic coating. Within thelaser cavity is an optical Q-switch 8 in the form of an acousto-opticmodulator. It is used as the shutter for establishing the operatingfrequency of the laser. Beyond the Q-switch is the output mirror 9. Thetwo mirrors, 7 on the pumped end of the laser rod and 9 beyond theacoustic optical Q-switch, comprise the laser cavity.

[0024] A system optical switch 13 in the form of a further acousto-opticattenuator is positioned beyond the laser cavity, in the laser outputbeam. Under control of computer 33, it serves both to prevent the beamfrom reaching the galvanometers except when desired, and, when the beamis desired at the galvanometers, to controllably reduce the power of thelaser beam to the desired power level. During vaporization proceduresthis power level may be as little as 10 percent of the gross laseroutput, depending upon operating parameters of the system and process.The power level may be about 0.1 percent of the gross laser outputduring alignment procedures in which the laser output beam is alignedwith the target structure prior to a vaporization procedure.

[0025] In operation, the positions of the X, Y galvanometers. 10 and 12are controlled by the computer 33 by galvanometer control G. Typicallythe galvanometers move at constant speed over the semiconductor deviceon the silicon wafer. The laser is controlled by timing signals based onthe timing signals that control the galvanometers. The laser operates ata constant repetition rate and is synchronized to the galvanometers bythe system optical switch 13.

[0026] In the system block diagram of FIG. 1 the laser beam is shownfocused upon the wafer. In the magnified view of FIG. 2, the laser beamis seen being focused on a link element 25 of a semiconductor device.

[0027] The metal link is supported on the silicon substrate 30 bysilicon dioxide insulator layer 32, which may be, e.g., 0.3-0.5 micronsthick. Over the link is another layer of silicon dioxide (not shown). Inthe link blowing technique the laser beam impinges on the link and heatsit to the melting point. During the heating the metal is prevented fromvaporizing by the confining effect of the overlying layer of oxide.During the duration of the short pulse, the laser beam progressivelyheats the metal, until the metal so expands that the insulator materialruptures. At this point, the molten material is under such high pressurethat it instantly vaporizes and blows cleanly out through the rupturehole.

[0028] The wavelength produced by wavelength shifter 12 is arrived at byconsidering on an equal footing the values of both the interconnect orlink to be processed and the substrate, in such a way as to trade-offenergy deposition in the substrate, which is undesirable, against energydeposition in the link structure, which is necessary to sever the link.Thus, the criteria for selecting the wavelength do not require thesubstrate to be very transparent, which is especially important if thewavelength regime in which the substrate is very transparent is muchless than optimal for energy deposition in the link structure.

[0029] The criteria for selection of the appropriate wavelength are asfollows:

[0030] 1) Calculate the relative incident laser beam energy required todeposit unit energy in the link structure This relative incident laserbeam energy is proportional to the inverse of the absorption of the linkstructure. For example, if the link structure has an absorption of0.333, it will require three times as much incident laser energy todeposit as much energy in the link structure as it would if thestructure had an absorption of 1. FIG. 3 illustrates absorption ofcopper, gold, aluminum, and silicon as a function of wavelength (copper,gold, and aluminum being possible link structure materials and siliconbeing a substrate material).

[0031] 2) Using the incident beam energy computed in step (1), calculatethe energy deposited in the substrate. For a well-matched laser spot,this energy will be proportional to the incident energy calculated instep (1), less the energy absorbed by the link structure, multiplied bythe absorption of the substrate. In other words, the energy absorbed inthe substrate is proportional to (1/L−1)×S (herein, “the substrateabsorption function”), where L is the absorption in the link and S isthe absorption in the substrate.

[0032] 3) Look for low values of the substrate absorption functiondefined in step (2) as a function of laser wavelength.

[0033]FIG. 4 illustrates the substrate absorption function for copper,gold, and aluminum links on a silicon substrate, as a function ofwavelength in the range of 0.3 to 1.4 μm. The values of the substrateabsorption function can be derived from the absorption curvesillustrated in FIG. 3, using a proportionality constant (see step (2)above) arbitrarily chosen as 0.5 for the sake of specificity (thisconstant merely changes the vertical scale of FIG. 4, and does not alterany conclusions drawn from it).

[0034] It can be seen from FIG. 4 that for structures of gold and copper(but not for aluminum) there is a region of wavelength less than roughly0.55 μm in which the substrate absorption function is comparable to thatin the region of wavelength greater than 1.2 μm.

[0035] It will also be noted that this function is quite different thanthe ones presented in FIGS. 5 and 6, which illustrate two possiblefunctions representing simple absorption contrast. More specifically,FIG. 5 illustrates the function L−S, expressed as percentage, and FIG. 6illustrates the function (L−S)/(L+S). In either case, the less-than−0.55 μm wavelength region is not found desirable according to FIGS. 5and 6, even for gold or copper link structures, because the functionshown in these figures is less than zero in this region. This negativevalue reflects the fact that the substrate is more absorptive than thelink structure in this wavelength regime, and so, according to thesemodels, this wavelength regime should not be selected.

What is claimed is:
 1. A method of vaporizing a target structure on asubstrate, comprising the steps of: calculating, as a function ofwavelength, an incident beam energy necessary to deposit unit energy inthe target structure; calculating, for the incident beam energy, energyexpected to be deposited in the substrate as a function of wavelength;identifying a wavelength corresponding to a relatively low value of theenergy expected to be deposited in the substrate, the low value beingsubstantially less than a value of the energy expected to be depositedin the substrate at a higher wavelength; providing a laser systemconfigured to produce a laser output at the wavelength corresponding tothe relatively low value of the energy expected to be deposited in thesubstrate; and directing the laser output at the target structure on thesubstrate at the wavelength corresponding to the relatively low value ofthe energy expected to be deposited in the substrate, in order tovaporize the target structure.
 2. The method of claim 1 wherein thewavelength corresponding to the relatively low value of the energyexpected to be deposited in the substrate is substantially less than1.047 μm.
 3. The method of claim 2 wherein the wavelength correspondingto the relatively low value of the energy expected to be deposited inthe substrate is less than 0.55 μm.
 4. The method of claim 3 wherein thetarget structure comprises a metal having a conductivity greater thanthat of aluminum.
 5. The method of claim 4 wherein the metal comprisescopper.
 6. The method of claim 4 wherein the metal comprises gold. 7.The method of claim 4 wherein the substrate comprises silicon.
 8. Themethod of claim 1 wherein the target structure on the substratecomprises a link of a semiconductor device.
 9. The method of claim 8wherein the semiconductor device comprises an integrated circuit. 10.The method of claim 8 wherein the semiconductor device comprises amemory device.
 11. The method of claim 1 wherein the energy expected tobe deposited in the substrate is substantially proportional to theincident beam energy necessary to deposit unit energy in the targetstructure minus the energy deposited in the target structure, multipliedby absorption of the substrate.
 12. A system for vaporizing a targetstructure on a substrate, comprising: a laser pumping source; a laserresonator cavity configured to be pumped by the laser pumping source;and a laser output system configured to produce a laser output fromenergy stored in the laser resonator cavity and to direct the laseroutput at the target structure on the substrate in order to vaporize thetarget structure, at a wavelength corresponding to a relatively lowvalue of energy expected to be deposited in the substrate, given anincident beam energy necessary to deposit unit energy in the targetstructure, the low value being substantially less than a value of theenergy expected to be deposited in the substrate at a higher wavelength.13. The system of claim 12 wherein the laser output system comprises awavelength shifter.
 14. The system of claim 12 wherein the laserresonator cavity produces laser radiation at the wavelengthcorresponding to the relatively low value of energy expected to bedeposited in the substrate.
 15. The system of claim 12 wherein thewavelength corresponding to the relatively low value of the energyexpected to be deposited in the substrate is substantially less than1.047 μm.
 16. The system of claim 15 wherein the wavelengthcorresponding to the relatively low value of the energy expected to bedeposited in the substrate is less than 0.55 μm.
 17. The system of claim16 wherein the target structure comprises a metal having a conductivitygreater than that of aluminum.
 18. The system of claim 17 wherein themetal comprises copper.
 19. The system of claim 17 wherein the metalcomprises gold.
 20. The system of claim 17 wherein the substratecomprises silicon.
 21. The system of claim 14 wherein the targetstructure on the substrate comprises a link of a semiconductor device.22. The method of claim 21 wherein the semiconductor device comprises anintegrated circuit.
 23. The method of claim 21 wherein the semiconductordevice comprises a memory device.
 24. The method of claim 14 wherein theenergy expected to be deposited in the substrate is substantiallyproportional to the incident beam energy necessary to deposit unitenergy in the target structure minus the energy deposited in the targetstructure, multiplied by absorption of the substrate.
 25. A method ofvaporizing a target structure on a substrate, comprising the steps of:providing a laser system configured to produce a laser output at thewavelength corresponding to a relatively low value of energy expected tobe deposited in the substrate, given an incident beam energy necessaryto deposit unit energy in the target structure, the low value beingsubstantially less than a value of the energy expected to be depositedin the substrate at a higher wavelength; and directing the laser outputat the target structure on the substrate at the wavelength correspondingto the relatively low value of the energy expected to be deposited inthe substrate, in order to vaporize the target structure.